Terrorism: Attendance at the five Conejo schools is down as much as 75% the day after the typed warning is sent. No explosive is found. Security heightened.

THOUSAND OAKS — Students stayed home in droves Thursday from five schools targeted this week in a typewritten bomb threat, which sheriff's officials now consider a hoax.

The Conejo Valley Unified School District said classroom attendance was down as much as 75% Thursday at the four elementary schools and one middle school that received copies of an anonymous letter Wednesday warning that a bomb would go off at one of the campuses.

"Why take the risk? There's a lot of nuts out there," said Susan Speidel, in explaining why she kept her son and daughter home from Los Cerritos Middle School.

Deputies searched Los Cerritos on Wednesday as well as the four other campuses: Ladera, Lang Ranch, Meadows and Weathersfield elementary schools.

Sweeps were also conducted at Park Oaks Elementary School, which was mailed a copy of the letter but was not named as a target. Attendance there was down 20%, said Assistant Supt. Richard Simpson.

"When somebody says we planted bombs and then we don't find any, that's a hoax," said Senior Deputy Ed Tumbleson.

School officials refused to confirm how long private security guards, brought in Wednesday evening, would remain on the campuses to avoid divulging information that might assist a would-be bomber.

Deputies increased patrols of the neighborhoods near the six campuses Thursday, and sheriff's officials said some heightened presence would continue through today.

Tumbleson said he did not know the intent of the letter writer and that the date of the threatened explosion, March 30, also did not register as particularly significant.

But Simpson said the letter did not appear to be written by an elementary or middle school student. He said high schools in the district occasionally receive bomb threats from students who would like to skip school for the day.

"When spring fever hits, it's not that unusual," he said. But, "to receive a anonymous terrorist threat and no high schools are involved, that's unusual."

Ingrid Orr was among the parents who accompanied their children to classes. She and her first-grade daughter, Alyssa, spent the day at Meadows Elementary School, though Alyssa was one of only three of the 20 students in her class to show up.

"I just thought that statistically it was unlikely that anything would happen," Orr said.